A study of the purine and pyrimidine metabolizing enzymes of a number of trypanosomid flagellates (Crithidia fasciculata, Leishmania donovani, L. braziliensis, L. mexicana and L. tarantolae) from a comparative point of view. Some of the key enzymes will be purified and characterized. Attention will be given to the mode of transport into the cell of the vases and ribonucleosides and the modifications which may take place in the presence of certain inhibitors.